1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus) for providing a tomographic image of an object to be examined at an examination position using nuclear magnetic resonance (NMR) phenomenon and in particular to an MRI apparatus in which deterioration in picture quality of a tomographic image can be prevented by reducing the influence due to a change in environmental magnetic field around magnetic field generating means.
2. Description of the Related Art
Conventional MRI apparatus has been magnetically shielded by enclosing the peripheral six sides of the whole of the apparatus with a high permeable material such as permalloy, amorphous metals for preventing the picture quality of a tomographic image from deteriorating by reducing the influences due to a change in environmental magnetic field around magnetic field generating means. However, in this case, cost of materials and setting work is expensive and prevention of deterioration in picture quality is not complete.
In order to overcome this, an MRI apparatus as described in JP-A-Hei 2-45035 has been proposed. As shown in FIG. 1, a change F in the environmental magnetic field around magnetic field generating means (not shown) is detected by a magnetic field sensor 31 and a signal detected by the magnetic field sensor 31 is amplified by an amplifier 32. A correcting current is generated by a correcting magnetic field coil power source 33 depending upon the detected signal and is conducted through looped correcting magnetic field coils 34 which face to each other at upper and lower positions in a magnetic circuit for generating a static magnetic field and are disposed in a parallel relationship with each other so that a correcting magnetic field F' is generated in a direction opposite to the change F in the environmental magnetic field. This correcting magnetic field F' will cancel the change F in the environmental magnetic field.
In such a conventional MRI apparatus, the correcting magnetic field coils 34 are wound on the outer periphery of disc-shaped magnetic cores for enhancing the uniformity of the static magnetic field in the magnetic circuit for generating the static magnetic field. Accordingly, when the correcting current I is conducted through the correcting magnetic field coils 34 as shown in FIG. 2A, eddy currents are generated in the magnetic cores and peripheral magnetic members. The induced correcting magnetic field F' is delayed in phase with respect to the correcting current I as shown in FIG. 2B. Therefore, the change F in the environmental magnetic field can not be completely suppressed even if the change F in the environmental magnetic field is corrected with the delayed correcting magnetic field F'. As a result of this, a magnetic field which is shifted in place due to the delay remains as a residual magnetic field.
In contrast to this, in order to minimize the foregoing delay, an MRI apparatus in which the correcting magnetic field coils 34 are provided externally of a magnetic circuit for generating the static magnetic field and a differentiating circuit for correction the delay of the correcting current supplied to the correcting magnetic field coils is provided in a correcting control circuit comprising the amplifier 32 and the correcting magnetic field coil power source 33 has been proposed as described in JP-A-Hei 3-280936. In this case, when a factor which causes the change F in the environmental magnetic field is, for example, a vehicle such as an electric train, an automobile or an elevator disposed externally of the apparatus causing a relatively slow change (direct current change), the delay of the correcting magnetic field F' is hardly influenced by provision of the correcting means. A sufficient effect is obtained.